A passive optical network (PON) features a point-to-multi-point (P2MP) architecture to provide broadband access. In particular, a single optical link has an optical splitter to branch optical signals to multiple premises.
The P2MP architecture has become the most popular solution for FTTx deployment among network operators. FTTx stands for “Fiber to the x”, which is a reference to any broadband network architecture using optical fiber to replace all or part of the metal local loop used for last-mile telecommunications.
PON-based FTTx has been widely deployed ever since 2004. Most of these PON systems are based on the standards of G-PON or E-PON. To meet the growing capacity demands driven by bandwidth-intensive applications, the 10 Gbps based PON technology (XG-PON1) was standardized in 2007 and is now ready for deployment to expand current communication capacities by factor of 4.
The FSAN Group (www.fsan.org) is now focusing on a future generation of access solutions named NG-PON2. This new technology, targeting standardization in 2012-2013 and commercial deployment in 2015, should be able to expand fiber capacity by another factor of 4 or more to achieve at least 40 Gbps downstream and 10 Gbps upstream. This will pave the way for provisioning of even higher bandwidth services in the future.
Among many proposals to meet the NG-PON2 criteria, wavelength-division multiplexing (WDM) stacked XG-PON is the only one that has been selected by FSAN to move into the standardization process. FIG. 1 shows the basic architecture 100 of this WDM stacked XG-PON proposal for NG-PON2. NG-PON2 introduced WDM signals into the system, where 4-16 wavelengths at 100 GHz spacing could be used for both down-stream and up-stream transmission. For a system with 4 wavelengths, a 1×4 cyclic demultiplexer or demux 102 is equipped in the central office (CO) 104 and is connected to 4 optical line termination (OLT) transceivers 106. For down-stream communications, the OLT transmitters are set to 4 fixed wavelengths in the down-stream band though each of the OLT transmitters could be wavelength tunable. At the optical network unit (ONU) side 108, a built-in tunable optical filter (TOF) is responsible for selecting one of the 4 wavelengths for the ONU receiver. For up-stream communications, the ONU transmitter should have wavelength tunability to communicate with any one of the 4 OLT receivers. To minimize the cost of an ONU transmitter, distributed feedback (DFB) lasers are used; wavelength tunability is achieved through thermal tuning. Since most DFBs have a tuning range of 3-4 nm where their center wavelength could distribute over a wider wavelength range, a 1×4 cyclic demux is a solution to connect each DFB laser to any OLTs it wants to reach by setting it to the right wavelength, as shown in FIG. 2.
Currently, cyclic demultiplexers are implemented using a cascade of 1×2 inter-leavers. That is, 1×2 inter-leavers are used as the building block for higher port count (1×4 or 1×8) cyclic demultiplexers. For example, a 1×4 cyclic demultiplexer can be achieved through 2 stage, 3 inter-leavers, though the two stages will have different free spectral ranges. This solution is relatively expensive and is otherwise not scalable.
Another architectural approach is arrayed waveguide grating. An arrayed waveguide grating can achieve 1×N or even N×N cyclic function on a monolithic chip. Unfortunately, current cyclic arrayed waveguide grating designs suffer higher insertion loss (>4 dB) and narrower bandwidth, particularly when the free spectral range is limited to 400 GHz-800 GHz.
Consequently, new techniques are required for cyclic demultiplexers. Ideally, such techniques will provide a low cost solution, low insertion loss and a wide passband.